1. Field of the Invention
The present invention relates to the treatment of vascular lesions using laser radiation.
2. State of the Art
The use of flashlamp pumped dye lasers has become the preferred form of treatment for a range of vascular lesions of the human body. In this treatment, a flashlamp is driven by a short pulse to excite or pump a laser light cell containing an appropriate dye solution, in order to emit a corresponding pulse of laser light at a predetermined wavelength, which is directed at the lesion to be treated; a preferred wavelength is 585 nm. Hitherto, it has been the accepted approach to maximise the temporal pulse length of the laser output pulse, with the aim of extending the period for which the target blood vessels are at elevated temperature and therefore increasing the probability that necrosis of the blood vessel (and hence permanent removal of the lesion) will occur. Accordingly, previous approaches have involved the use of laser output pulses of some 450 to 500 microseconds duration.
We have now devised an alternative approach, in the use of flashlamp pumped dye lasers, which results in more effective treatment of vascular lesions.
In accordance with this invention as seen from one aspect, there is provided a flashlamp pumped dye laser system which comprises:
(a) a dye laser;
(b) one or more flashlamps for exciting the dye laser; and
(c) at least one pulse generating circuit for driving the or each said flashlamp, the pulse generating circuit being configured so as to provide a driving current having a risetime of less than 100 microseconds arranged to produce corresponding laser output pulses from the dye laser having a risetime of less than 100 microseconds.
Thus, we have discovered that a more effective treatment of vascular lesions can be achieved by significantly reducing the risetime of the laser output pulse which is most conveniently achieved using a driving circuit in which the current rises to a maximum rapidly, whereby the laser output follows closely the risetime of the driving current pulse. Hitherto, the laser output pulse has exhibited a risetime generally of 200 to 250 microseconds, the overall pulse duration being of the order of 450 to 500 microseconds. In systems which allegedly produce a rapid laser pulse risetime, specially configured laser arrangement and driving circuitry have been proposed (such as for example the arrangement disclosed in U.S. Pat. No. 5,287,380 which proposes a ramp pulse rising continuously throughout the laser pulse). The present invention provides the required performance in a simple and elegant manner.
Preferably, in the system in accordance with the present invention, the laser output pulse (and also preferably the drive current pulse) has risetime of less than 80 microseconds. We believe that there is an optimum risetime within the range of 30 to 80 microseconds and most preferably at substantially 50 microseconds. Preferably the laser output pulse maintains its maximum output level for 120 to 1000 microseconds, preferably 120 to 200 microseconds, and most preferably 150 microseconds.
Use of the system of the present invention extends the period for which the target blood vessel will be above a threshold temperature, so that necrosis can occur. Also, the significantly faster rise in temperature of the target blood vessel produces a faster increase of pressure due to local blood vaporisation, thus increasing the probability of vessel rupture and hence necrosis.
Also, in accordance with the present invention, as seen from a second aspect, there is provided a method of cosmetic treatment of vascular lesions, comprising directing pulses of radiation from a flashlamp pumped dye laser toward the lesions, the laser output pulses having a risetime of less than 100 microseconds. Preferably, the laser output pulse has a risetime of less than 80 microseconds. There is an optimum risetime within the range of 30 to 80 microseconds and most preferably at substantially 50 microseconds.
It is preferred that the laser output pulse maintains its maximum output level at a pulse duration in the range 120 to 1000 microseconds, preferably 120 to 200 microseconds, and most preferably 150 microseconds.
In systems which have been used hitherto, it has been known to use a drive circuit in which one side of the flashlamp is connected through a transformer secondary winding and a resistor to a high voltage supply, a capacitor is connected from the resistor to the ground side of the circuit, and the other side of the flashlamp is connected through an inductor to the ground side. A short trigger pulse is applied to the transformer primary winding, so that a pulse of radiation is emitted from the flashlamp. The inductor in the ground side circuit of the flashlamp has the effect of extending the pulse duration.
In accordance with the present invention as seen from a third aspect, there is provided a flashlamp drive circuit, comprising at least one flashlamp having one side connected through a transformer secondary winding, and a resistor to a high voltage supply, a capacitor connected from the resistor to the ground side and the flashlamp having its other side connected directly to the ground side circuit. It is preferred that two such circuits are provided, connected to each other.
In this circuit, because of the substantial absence of any inductance in the ground side circuit of the flashlamp, the pulse which drives the flashlamp exhibits a substantially reduced risetime. The circuit is configured in such a way that the rising portion of the current pulse is substantially unrestricted leading to a relatively rapid risetime to a peak value before subsequently tailing off. The rise time of the driving current pulse in U.S. Pat. No. 5,287,380 is controlled so as to be increasing throughout the duration of the laser output pulse.